Field of Invention
The present invention generally relates to the field of semiconductor wafer processing. More particularly, the present invention is directed to a wafer chuck having a removable insert.
During various steps of processing a semiconductor wafer during the manufacturing of microelectronic devices, the wafer must be held firmly in a fixed position with its processed surface as planar as possible. For example, during projection photolithography processing, the processed surface must be substantially coincident with the focal plane of the projection optics. If one or more regions of the processed surface deviate from the focal plane, the deviation must not exceed the depth of focus of the projection optics. Otherwise, the projected pattern at such regions will be distorted and the regions may be unusable.
As feature sizes of patterns are becoming increasingly smaller, so are depths of focus. This is so because smaller feature sizes require larger numerical apertures that translate into shallower depths of focus. In addition, the size of the field printed or scanned during each exposure is relatively large to increase throughput and increase productivity. These characteristics of current photolithographic practices accentuate the need for maintaining the processed surface of a wafer as planar as possible during photolithography.
Dirt particles on the backsides of wafers often present a significant obstacle in achieving the necessary planarity required for photolithographic patterning. Even with ultra-clean rooms and extensive wafer cleaning procedures, it is virtually impossible to completely rid the wafers, particularly their backsides, of dirt particles. In fact, many dirt particles that interfere with planarizing the processed surfaces of the wafer come from the wafers themselves after they have been cleaned. For example, such dirt particles are often in the form of chips from the wafer edges and flakes from films grown or deposited upon the wafers.
To minimize the impact of dirt particles on planarizing the processed surface, the wafer chucks used to hold wafers during photolithography have evolved from chucks having optically-flat surfaces for engaging the backside of a wafer to vacuum chucks having annular grooves in their optically-flat surfaces to hold the wafer firmly in place and, most recently, to pin-type, or xe2x80x9cbed-of-nails,xe2x80x9d vacuum chucks, which are generally represented by prior art vacuum chuck 10 illustrated in FIGS. 1 and 2.
As shown in FIGS. 1 and 2, prior art vacuum chuck 10 comprises a metal base 12 having an upper surface 14 and a plurality of aluminum pins 16 extending upward from the upper surface. The upper ends 18 of the plurality of pins 16 all lie within a common plane that is positioned parallel to the focal plane of the projection optics. The upper end 18 of each pin 16 has a planar surface 20 that engages a wafer 22. If wafer 20 is warped or otherwise not planar, it may not engage one or more of pins 16. To reduce or eliminate any out-of-planeness of wafer 22 and/or to hold the wafer firmly to chuck 10, the chuck is connected to a vacuum source (not shown) that provides a vacuum in a vacuum region 24 located between wafer 22 and upper surface 14 of chuck vacuum 10. Vacuum region 24 is in fluid communication with the vacuum source via vacuum ports 26 and is generally isolated from the environment surrounding vacuum chuck 10 when wafer 20 is inserted therein by a seal ring 28.
Although pin-type vacuum chucks, such as vacuum chuck 10 described above, are a vast improvement over chucks having larger areas of contact with wafers, they are still subject to the influence of dirt particles and, therefore, must be cleaned periodically to prevent the dirt particles from affecting the lithography process. Over time, continual cleaning of vacuum chuck 10 typically causes upper ends 18 of pins 16 to be worn away, particularly at the pins proximate to the center of the chuck. Uneven wear among pins 16 causes the upper ends 18 of the pins to define a non-planar, rather than planar, surface, which reduces the accuracy of vacuum chuck 10, and eventually causes intolerable focus error that requires the chuck to be replaced. Another disadvantage of vacuum chuck 10 is that the wafer contact area of planar surface 20 of each pin 16 is relatively large, increasing the probability that dirt particles on wafer 22 will interfere with direct contact between the wafer and one or more pins.
In one aspect, the present invention is directed to a chuck/insert assembly for holding a workpiece. The chuck/insert assembly comprises a chuck that includes a first base having a first surface. At least one first spacer extends away from the first surface. An insert is removably engaged with the at least one first spacer so as to form a first vacuum region between the insert and the chuck. The insert includes a second base having a second surface. At least one second spacer extends away from the second surface and has a distal end located opposite the second surface. The distal end is adapted for engaging the workpiece.
In another aspect, the present invention is directed to a method of forming an insert for a vacuum chuck from a body having a surface. The method comprises the steps of patterning onto the surface of the body at least one spacer location and removing material surrounding the at least one spacer location so as to form at least one spacer.